Mobile satellite communications system with local and global network

ABSTRACT

A communication system for communication with a plurality of mobile terminals. A plurality of earth stations are arranged to communicate with mobile users via a plurality of orbiting satellites. A plurality of gateway stations interconnect terrestrial equipment with the earth stations and a store stores access data for the mobile terminals. The mobile terminals are divided into first and second categories, the store retaining corresponding different status information and both the first and second categories communicating with the satellites. Also provided are first and second different communication channels between the gateway stations and the earth stations, and a route control system for selecting one of the first and second channels, in dependence upon the category of a mobile terminal.

FIELD OF THE INVENTION

This invention relates to communications with a mobile user, and inparticular to such communications in which the link to the mobile useris via a satellite or satellites.

BACKGROUND ART

US 4189675 proposes a satellite communications method and apparatus forcommunicating with mobile users using a network of satellite inpredetermined orbits. It would be possible to provide a completecommunications network using only satellites, by utilisingsatellite-to-satellite links. However, both power and bandwidth are at apremium in satellite communications, and the capacity of such a networkwould therefore be small. It has therefore been proposed to employground components to form part of the link between one user and another.

A problem which arises is that of knowing where a mobile user is, inother words, to which satellite (and, with multi beam satellites, whichbeam) a call to that user should be directed. This problem is similar tothat arising in the context of international terrestrial cellular radiosystems, such as the GSM system. In that system, a register ismaintained in a store of the locations of mobile users.

EP 0562374 and EP 0568778 are believed to describe the call set upmethod used in the “Iridium” proposed satellite cellular mobilecommunication system. They describe a network in which information onthe position of a mobile user is maintained at the ground switchingoffice which is currently serving the mobile user, and also at a “home”station for each particular mobile subscriber. When a call is placed toa particular mobile subscriber, either the satellite switching office towhich the call is first routed is the/one currently serving thesubscriber (in which case the call is placed directly via the satelliteto the subscriber), or the switching office contacts the “home” stationfor the subscriber, and obtains the details of the current switchingoffice serving the subscriber, to which the call is routed (although itis not clear in what manner the routing occurs).

An alternative description of the “Iridium” proposals is given in thepaper “The Iridium (TM) system personal communications anytime, anyplace” J. E. Hatlelid and L. Casey, Proceedings of the ThirdInternational Mobile Satellite Conference IMC 93, 16-18 June 1993, pages285-290, which reveals that it is proposed that the Iridium satellitecellular system is intended to operate with terrestrial cellular radio,and that calls will be transmitted via a satellite only if terrestrialcellular communication is not available.

An alternative proposed satellite cellular system is described in “TheGlobalstar Mobile Satellite System for Worldwide PersonalCommunications”, R. A. Weideman, pages 291-296 of the ConferenceProceedings mentioned above. Two alternative access network schemes,invented by the author of that paper, and believed to have been proposedfor use in the proposed Globalstar system, are described in EP 0536921and EP 0506255. In the former of these, each mobile user is allocated a“home” gateway station, containing information on that user.Additionally, each gateway contains information on all mobile userscurrently in its area.

Calls are directed to the users home gateway station. If this is thegateway station within the area of which the user is currently located,the call is then routed to the mobile user by satellite. If not, thedatabase held at the home gateway station is consulted and the call isre-routed from the home gateway to the active gateway within the area ofwhich the mobile user is located. Details of the rerouting are notgiven; it is presumably via the public network.

EP 0506255 is similar, but the user information and processingintelligence is located in the satellites, rather than in groundstations.

Both these latter two documents propose to use the satellite link onlywhen a user has registered as “roaming” outside his normal area ofoperations; whilst inside this area, he is served by a terrestrialcellular communications network.

None of the foregoing publications take account of the technicalproblems which may arise when a satellite cellular communications systemis used for connection to a roaming mobile user who may be located atone of a number of widely dispersed points on the Earth, and who may becalled from a fixed telephone at any point across the earth via a publicswitched telephone network (PSTN).

To give an example, suppose that in the system described in EP 0536921,a mobile user has a home gateway in Australia, but is roaming in theUnited Kingdom (i.e. on a diametrically opposed point on the planet),and that he is called from a user in the UK. The call would first berouted through the international PSTN from the UK to Australia, to thehome gateway, whence it would be re-routed by a further link back to anactive gateway in or near the UK, then finally relayed via the satelliteto the mobile user. When it is borne in mind that each of the linksbetween the UK and Australia may involve at least one satelliteconnection via, for example, satellites operated by INTELSAT, it will beseen that at least three “hops”, each comprising anearth-satellite-earth round trip are involved. The total delay in thelink thus can rapidly become very substantial, with consequent rapiddegradation in the perceived quality of the connection. Further, othertypes of noise, distortion or echo degradation of the signal may alsocome into play.

SUMMARY OF THE INVENTION

The present invention provides a satellite mobile communications systemin which a plurality of terrestrial gateways are interconnected by adedicated terrestrial ground network; calls to mobile users in acategory of users who are entitled to roam globally are routed throughthe dedicated network to the ground station serving the satellitecurrently serving the mobile user; and calls directed to a mobile userwho is in a category of users not entitled to roam outside a limitedgeographical area are directed to the ground station serving thatgeographical area via another network (e.g. the PSTN on which the callarrived).

Thus, for mobile users who may genuinely be located at dispersedlocations over time, the call is routed via a dedicated, controllednetwork which may, for example, consist entirely of terrestrial links,so that the quality of the link to the earth station serving thesatellite serving the mobile user can be controlled.

On the other hand, users who will not require global roaming (and whoseposition is therefore known) can be served predominantly through -thePSTN, since a relatively direct and short route through the publicswitched telephone network can be set up.

According to another aspect of the invention, there is provided a methodof interconnecting a satellite mobile communications system userterminal and another telecommunications user terminal, comprisingselecting one from a plurality of possible earth stations via which saidsatellite system user terminal may communicate via a satellite, andinterconnecting the two user terminals via said selected earth station.

The selection of the appropriate ground station may, in one embodiment,be made based on a prediction of link quality taking into account thesatellite system user position and the ephemerides of the satellitesserving each earth station. It may equally be made taking into account,additionally or alternatively, the measured quality or strength of thelink obtainable via each of a plurality of earth stations.

In an embodiment, where several earth stations provide an acceptablelink quality, the choice may be made so as to minimise the route to betaken through the ground network (e.g. by making the selected earthstation the same as the source or destination earth stations of anincoming or outgoing call, respectively) or to maximise quality throughthe ground network (e.g. by avoiding long distance links such assatellite links where possible).

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings, in which:

FIG. 1 is a block diagram showing schematically the elements of acommunications system embodying the present invention;

FIG. 2 is a block diagram showing schematically the elements of mobileterminal equipment suitable for use with the present invention;

FIG. 3 is a block diagram showing schematically the elements of an Earthstation node forming part of the embodiment of FIG. 1;

FIG. 4 is a block diagram showing schematically the elements of agateway station forming part of the embodiment of FIG. 1;

FIG. 5 is a block diagram showing schematically the elements of adatabase station forming part of the embodiment of FIG. 1;

FIG. 6 illustrates the contents of a store forming part of the databasestation of FIG. 5;

FIG. 7a illustrates schematically the beams produced by a satellite inthe embodiment of FIG. 1;

FIG. 7b illustrates schematically the disposition of satellites formingpart of FIG. 1 in orbits around the earth;

FIG. 8 illustrates the geographical disposition of the components ofFIG. I on the Earth;

FIG. 9 corresponds to a part of FIG. 8 and shows the path taken by anincoming call to a locally registered mobile terminal;

FIG. 10 corresponds to FIG. 9 and shows the corresponding path taken byan outgoing call from the mobile terminal;

FIG. 11 corresponds to FIG. 9 and shows the path taken by aninternational incoming call to a locally registered mobile terminal;

FIG. 12 corresponds to a part of FIG. 8 and shows the path taken by anincoming call to a globally registered mobile terminal;

FIG. 13 corresponds to FIG. 12 and shows the corresponding path of anoutgoing call from the mobile terminal;

FIG. 14 illustrates schematically four possible paths between a mobilestation, two satellites and two earth stations;

FIG. 15 corresponds to FIG. 14 with the addition of a shadowingobstacle;

FIG. 16, consisting of FIGS. 16a and 16 b, is a flow diagramillustrating the process of allocation of an active earth station for acall to a mobile user; and

FIG. 17, consisting of FIG. 17a and 17 b, is a flow diagram illustratingthe process of allocation of an active earth station for a call directedto a mobile user.

PREFERRED EMBODIMENT

Referring to FIG. 1, a satellite communications network according tothis embodiment comprises mobile user terminal equipment 2 a, 2 b;orbiting relay satellites 4 a, 4 b; satellite earth station nodes 6 a, 6b; satellite system gateway stations 8 a, 8 b; public switchedtelecommunications networks 10 a, 10 b; and fixed telecommunicationsterminal equipment 12 a, 12 b.

Interconnecting the satellite system gateways 8 a, 8 b with the earthstation nodes 6 a, 6 b, and interconnecting the nodes 6 a, 6 b with eachother, is a dedicated ground-based network comprising channels 14 a, 14b, 14 c. The satellites 4, earth station nodes 6 and lines 14 make upthe infrastructure of the satellite communications network, forcommunication with the mobile terminals 2, and accessible through thegateway stations 8.

A terminal location database station 15 is connected, via a signallinglink 60 (e.g. within the channels 14 of the dedicated network) to thegateway station and earth stations 6.

The PSTNs 10 a, 10 b comprise, typically, local exchanges 16 a, 16 b towhich the fixed terminal equipment 12 a, 12 b is connected via localloops 18 a, 18 b; and international switching centres 20 a, 20 bconnectable one to another via transnational links 21 (for example,satellite links or subsea optical fibre cable links). The PSTNs 10 a, 10b and fixed terminal equipment 12 a, 12 b (e.g. telephone instruments)are well known and almost universally available today.

Each mobile terminal apparatus is in communication with a satellite 4via a full duplex channel (in this embodiment) comprising a down linkchannel and an up link channel, for example (in each case) a TDMA timeslot on a particular frequency allocated on initiation of a call, asdisclosed in our earlier UK patent applications GB-A-2288913 andGB-A-2293725. The satellites 4 in this embodiment are non geostationary,and thus, periodically, there is hand over from one satellite 4 toanother.

Mobile terminal 2

Referring to FIG. 2, the mobile terminal equipment of FIG. 1 is shown.

One suitable form is a handset, as shown. Details of the handsets 2 a, 2b etc do not form part of the present invention, but they may comprisehandsets similar to those presently available for use with the GSMsystem, comprising a digital coder/decoder (CODEC) 30, together withconventional microphone 36, loudspeaker 34, battery 40 and keypadcomponents 38, and a radio frequency (RF) interface 32 and antenna 31suitable for satellite communications. Preferably a display 39 (forexample a liquid crystal display) is also provided. A ‘smart card’reader receiving a smart card storing user information may be present.

Earth Station Node 6

The earth station nodes 6 are arranged for communication with thesatellites.

Each earth station node 6 comprises, as shown in FIG. 3, a conventionalsatellite earth station 22 consisting of at least one satellite trackingantenna 24 arranged to track at least one moving satellite 4, RF poweramplifiers 26 a for supplying a signal to the antenna 24, and 26 b forreceiving a signal from the antenna 24; and a control unit 28 forstoring the satellite ephemeris data, controlling the steering of theantenna 24, and effecting any control of the satellite 4 that may berequired (by signalling via the antenna 24 to the satellite 4).

The earth station node 6 further comprises a mobile satellite switchingcentre 42 comprising a network switch 44 connected to the trunk links 14forming part of the dedicated network. A multiplexer 46 is arranged toreceive switched calls from the switch 44 and multiplex them into acomposite signal for supply to the amplifier 26 via a low bit-rate voicecodec 50. Finally, the earth station node 6 comprises a local store 48storing details of each mobile terminal equipment 2 a within the areaserved by the satellite 4 with which the nodes 6 is in communication.

Gateway 8

Referring to FIG. 4, the gateway stations 8 a, 8 b are, in thisembodiment, commercially available mobile switch centres (MSCs) of thetype used in digital mobile cellular radio systems such as GSM systems.They could alternatively comprise a part of an international or otherexchange forming one of the PSTNs 10 a, 10 b operating under softwarecontrol to interconnect the networks 10 with the satellite system trunklines 14.

The gateway stations 8 comprise a switch 70 arranged to interconnectincoming PSTN lines from the PSTN 10 with dedicated service lines 14connected to one or more Earth station nodes 6, under control of acontrol unit 72. The control unit 72 is capable of communicating withthe data channel 60 connected to the database station 15 via asignalling unit 74, and is arranged to generate data messages in somesuitable format (e.g. as packets or ATM cells).

Also provided in the gateway stations 8 is a store 76 storing billing,service and other information relating to those mobile terminals 2 forwhich the gateway station 8 is the home gateway station. Data is writtento the store 76 by the control unit 72 after being received via thesignalling unit 74 or switch 70, from the PSTN 10 or the Earth stationnodes 6 making up the satellite network.

The satellite system trunk lines 14 comprise, in this embodiment, highquality leased lines meeting acceptable minimum criteria for signaldegradation and delay. In this embodiment, all the lines 14 compriseterrestrial links. The trunk lines 14 are preferably dedicated lines, sothat the lines 14 form a separate set of physical channels to thenetworks 10. However, the use of virtual circuits through the networks10 is not excluded.

Location Database Station 15

Referring to FIG. 5 the global database station 15 comprises a digitaldata store 54, a signalling circuit 56, a processor 58 interconnectedwith the signalling circuit 56 a and the store 54, and a signalling link60 interconnecting the database station 15 with the gateway stations 8and Earth stations 6 making up satellite system network, for signallingor data message communications.

The store 54 contains, for every subscriber terminal apparatus 2, arecord showing the current status of the terminal 2 (whether it is“local” or “global” as will be disclosed in greater detail below); thegeographical position of the mobile terminal 2 (either in co-ordinategeometry, or as code identifying an area within which it lies); the“home” gateway station 8 with which the apparatus is registered (toenable billing and other data to be collected at a single point) and thecurrently active Earth station node 6 with which the apparatus 2 is incommunication via the satellite 4. The contents of the store areindicated in FIG. 6.

The signalling unit 56 and processor are arranged to receiveinterrogating data messages, via the signalling circuit 60 (which may bea packet switched connection), from gateways 8 or nodes 6, comprisingdata identifying one of the mobile terminals 2 (for example, thetelephone number of the equipment 2), and the processor 58 is arrangedto search the store 54 for the status and active earth station node 6 ofthe terminal 2 and to transmit these in a reply message via the dataline 60.

Satellites 4

The satellites 4 a, 4 b comprise generally conventional communicationssatellites, and may be as disclosed in our earlier filed UK applicationGB-A-2288913. Each satellite 4 is arranged to generate an array of beamscovering a footprint beneath the satellite, each beam including a numberof different frequency channels and time slots, as described in ourearlier UK application GE-A-2293725 and illustrated in FIG. 7a.

The satellites 4 a are arranged in a constellation in sufficient numbersand suitable orbits to cover a substantial area of the globe (preferablyto give global coverage) for example 10 (or more) satellites may beprovided in two (or more) mutually orthogonal intermediate circularorbits at an altitude of, for example, 10,500 kilometres as shown inFIG. 7b. Equally, however larger numbers of lower satellites may beused, as disclosed in EP 0365885, or other publications relating to theIridium system, for example.

Geographical Arrangement

Referring to FIG. 8, one exemplary geographical arrangement is shown(only one satellite 4 a is shown for clarity). The database station 15may be located anywhere, and the position indicated is purely notionalin practice, it is preferably located in the geographical area fromwhich most calls originate, or within which the best signallinginfrastructure exists. In FIGS. 8 onwards, a dotted line denotes asignalling link and a solid line denotes a communications traffic link.

With the number of satellites mentioned below, there may typically be 12earth station nodes 6, two for each continents In this manner, eachearth station node 6 is connectable to gateways 8 on the continent,whilst having in view one or more satellites 4.

In this embodiment there are significantly larger number of gateways 8than of earth station nodes 6; on the order of one per country (i.e.over a hundred in total). For larger countries, several gateways 8 maybe provided at different geographical locations, or (where severalnetwork operators are permitted in the country) one per PSTN. Smallercountries may share gateways 8. Gateways 8 may also be provided fromterrestrial cellular networks (although this is not explicitly indicatedin FIG. 8).

For ease of explanation PSTNs 10 are omitted from FIG. 8 since theircoverage is virtually global.

The operation of the above embodiment will now be described in greaterdetail.

Registration and Location

In the present embodiment, as mentioned above, a customer mobileterminal apparatus 2 may be registered with one of two distinctstatuses; “local” in which the mobile terminal apparatus is permittedonly to communicate through one local area, or part of the satellitesystem network, and “global”, which entitles the apparatus tocommunicate through any part of the satellite system network. Since thelatter option places a greater demand on network resources (as will bediscussed below), a higher charge may be made for calls for customersregistered as “global” or “roaming” users.

The status of each apparatus 2 (i.e. “local” or “global”) is stored inthe record held for the apparatus 2 concerned in the store 54 of thedatabase station 15, either as a result of a manual input via a keyboardto the store acting on the expressed preference of the owner of theapparatus 2, or (as will be discussed below) in response to a signalfrom the apparatus 2 requesting a status change.

The mobile terminal apparatus 2 performs an automatic registrationprocess, of the kind well known in the art of cellular terrestrialcommunications, on each occasion when the terminal 2 is utilised for anoutgoing call; and/or when the apparatus 2 is switched on; and/orperiodically whilst the apparatus 2 is switched on. As is conventional,the registration process takes the form of the broadcasting of a signalidentifying the mobile terminal 2 (e.g. by transmitting its telephonenumber on a common hailing or signalling frequency).

The transmitted signal is picked up by one or more satellites 4. Undernormal circumstances, the signal is picked up by multiple satellites 4,and the received signal strength and/or time of arrival are transmitted,together with the identity of the mobile apparatus 2 and of thesatellite 4 receiving the signal, to the database station 15 via theearth stations node or nodes 6 for which the satellites 4 are incommunications, and the signalling line 60.

The processor 58 of the database station 15 then calculates, e.g. on thebasis of the differential arrival times, the terrestrial position of themobile terminal apparatus 2, which is stored in the database 54. Alsostored is the identity of the earth station node 6 most suitable forcommunicating with the mobile terminal apparatus 2 (the “active”station). This is typically found by the processor 58 comparing thestored position of the terminal 2 with the predetermined storedpositions of each of the earth station nodes 6 and selecting thenearest. However, account may also or instead be taken of the strengthof the signals received via the satellites 4, or of other factors (suchas network congestion) to result, in borderline cases, in the choice ofa node earth station which is not geographically closest to the mobileterminal equipment 2. The identity of the allocated active earth stationnode 6 is then likewise stored in the store 54 in the record for thatterminal apparatus.

LOCAL STATUS

The stored status of the mobile terminal equipment 2 is read by theprocessor 58. If the status is “local”, the stored position of themobile terminal apparatus is compared with the local range permitted tothe user (for example, the national boundaries of a country or a PSTN10). In a simple example according to this embodiment, the local area isa geographical area covered by a PSTN 10 connected to the home gatewaystation of the mobile terminal 2.

If the position of the terminal equipment 2 lies outside the local areawithin which it is permitted to communicate, an “out-of area” flag isset in the record in the store 54 relating to the user. A message istransmitted by the processor 58 (via the signalling circuit 56 andsignalling line 60) to the active earth station node 6 to which themobile terminal apparatus 2 has been allocated, comprising the identityof the mobile terminal unit 2, together with its geographical position,an indication that it is registered as a local user, an indication ofits “home” gateway, and a flag indicating that it is unavailable asbeing out of its area. This data is stored in a record created for themobile terminal equipments in the store 48 within the earth station node6.

The active earth station node 6 will take no further action to connecteither incoming or outgoing calls with the mobile apparatus 2. It maysend an “out of area” message via the satellite 4 to the mobileapparatus 2, comprising the identification code for the mobile and an“out of area” code, in response to which the mobile terminal apparatus 2may display a message on the display 39, or otherwise indicate the “outof area” status to a user (for example, by generating a audio tonethrough the loudspeaker 34 or illuminating an indicator lamp).

In the event that the processor 58 determines that the mobile terminalequipment 2 is registered as “local” and is within its permitted localarea, a message is transmitted to the active allocated earth stationnode 6 indicating the same details of identity, position and localstatus, but setting an “available” flag rather than an “out of area”flag as previously. In this instance, the earth station node 6 will infuture treat the mobile terminal apparatus 2 as being available forcommunications, by allocating a communications channel and satellite tothe apparatus on initiation of either an incoming or an outgoing call.

GLOBAL STATUS

If the processor 58 identifies, from the record in the store 54, thatthe status of the registering mobile terminal apparatus is “global”, ittransmits, via the signalling circuit and signalling line 60, a message(which, as above, contains the terminal identification number, status(“global”), position, and home gateway identification for the mobileterminal apparatus 2) to the allocated active earth station node 6,which creates or updates a record in its store 48.

Thus, as a result of the registration process described, a centralrecord for every mobile terminal apparatus 2 is maintained in the store54 of the database station 15 accessible by nodes 6 and gateways 8; andeach node 6 maintains in its store 48 a local record of every mobilecommunications apparatus 2 within its area, together with its positionand home gateway, and available or unavailable status.

It would also be possible for the stores 48 of each node 6 to store onlydata relating to global status and local—in—area status apparatus, andnot store data relating to local status mobile terminal 2 which are outof their own area.

The store 54 acts somewhat in the manner of the Home Location Register(HLR) of a GSM terrestrial cellular system, and the store 48 in themanner of the Visiting Location Register (VLR) of GSM; commerciallyavailable HLR and/or VLR equipment may therefore be employed for thesecomponents, modified as necessary.

CHANGE OF STATUS

The operator of a given mobile terminal apparatus 2 may change thestatus thereof between “local” and “global” or vice versa, either bycommunicating with the operator of the satellite system network andthereby causing a manual data entry into the contents of the store 54,or by initiating a sequence of key strokes on the key pad 38, orotherwise, inputting a signal to the mobile terminal apparatus 2.

In the former case, the change of status is signalled by the processor58 to the allocated active node 6, which updates the record within itsstore 48, and in the later case the sequence of key strokes receivedfrom the mobile terminal equipment 2 is relayed via the satellite 4 tothe active node 6, which transmits the change of status as a signal tothe database station 15, at which the processor 58 updates the recordstored in the store 54 and signals back for the active node 6 to dolikewise within its store 48. It is preferred that handshakingtechniques of this type be employed, to ensure that the records storedin the central store 54 and stores 48 in node stations 6 be kept inagreement.

Although it is not germane to the present invention, it will be apparentthat the stores 54 or 48 may store other information pertaining to thecustomer or apparatus 2, and that all information stored in the storemay be updated by manual entry or signalling from other sources (forexample, position location apparatus within mobile terminal equipment 2where available, using e.g. the Global Positioning System (GPS)). Anychange in information stored in relation to the customer or theequipment 2 is relayed to the central database station 15 in the form ofa message via the signalling link 60, and is distributed therefrom tothe active node 6.

In particular, further information is held at the “home” gateway 8,comprising at least billing records for usage of the satellite systemnetwork by the mobile terminal apparatus 2.

CALL SET UP AND ROUTING

The processes of routing calls to and from mobile terminal apparatus 2in the present invention will now be described. By way of example, callsto and from fixed stations 12 available through PSTN networks aredescribed, but it will be immediately apparent that the same principlesare applicable to communication to and from terrestrial mobile terminalsavailable through public land mobile networks (PLMNs).

1. LOCAL USER—LOCAL CALL

A. Fixed to Mobile Calls

First, the case where the mobile terminal equipment 2 is registered as a“local” user will be considered with reference to FIG. 9.

The terminal apparatus 2 a, being registered as “local” to the nearestgateway 8 a, has a dial number which is recognised by the PSTN 10 a towhich the gateway 8 a is connected as relating to the satellite servicenetwork.

The fixed terminal 12 a dials through the local PSTN node 16 a, and thecall is routed via the PSTN 10 a to the local gateway 8 a. The gateway 8a sends an interrogation message to the database station 15, comprisingthe dial number of the mobile terminal equipment 2 a. The databasestation 15 transmits back, in reply, the information that the terminalequipment 2 a is registered as local and is available (i. e. iscurrently in the local area of the gateway 8 a), and, in thisembodiment, the identification of the node 6 a via which the mobileterminal equipment 2 a is contactable.

The gateway 8 a sets up a connection, via ground network line 14 a, tothe ground station node 6 a, passing on the dial of the mobile terminalequipment 2 a to be called. The station node 6 a switches the call intothe appropriate multiplexed time frequency channel within the up linkbroadcast to the satellite 4 a (and, preferably, at least one furthersatellite 4 b, as disclosed in our earlier application GB-A-2293725.

The satellite 4 a demultiplexes the up link signal and routes the callto a time/frequency slot on one of its beams, via which it is broadcastto the mobile terminal equipment 2 a. A corresponding return link fromthe mobile station 2 a is set up, and the channel is held open duringthe duration of the call. The earth station node 6 a then transmitsbilling information to the local (home) gateway 8 a, for subsequentbilling of the mobile terminal 2 a.

B Mobile to Fixed Call

When a locally registered mobile terminal 2 a, which is within itspermitted area, wishes to initiate an outgoing call to a fixed station12 b, as shown in FIG. 10 the terminal 2 a transmits a message to theground station node 6 a via the satellite 4 a in including the dialnumber of the fixed terminal 12 a.

The earth station node 6 a transmits an interrogation message to thedatabase station 15, to determine the home gateway 8 a with which themobile terminal equipment 2 a is registered, and sets up a call to thehome gateway 8 a via the ground network line 14 a, through which thedial number of the fixed terminal 12 a is transmitted. The gateway 8 apasses the dial number to the local PSTN 10 a to which it is connected,and the call is thereby set up.

The circuit through the line 14 and PSTN 10 a is maintained during theduration of the call. After completion of the call, the earth stationnode 6 a transmits billing information to the home gateway 8 a as above.

2. LOCAL USER—INTERNATIONAL CALL

A Fixed to Mobile Call

Referring to FIG. 11, a fixed terminal 12 b in a different countrydials, as before, the dial number of a mobile terminal 2 a. The localexchange 16 b and PSTN lob recognise the prefix as being a call to theinternational satellite network and route the call to the local gatewaystation 8 b connected to the PSTN lob.

The gateway 8 b transmits an interrogation signal to the databasestation 15 including the dial number of the called mobile terminal 2 a.In reply, the database station 15 returns the information that themobile terminal 2 a is registered as a local terminal, and supplies thedial number of the home gateway 8 a of the mobile terminal 2 a.

The gateway 8 b holds open the connection from the PSTN 10 a, and setsup a return connection to the PSTN 10 a through which the dial number ofthe home gateway 8 a is supplied. The dial number is recognised as aninternational call by the PSTN 10 b and is routed, via internationalswitching centres 20 b, 20 a and transnational link 21, to the homegateway 8 a, which sets up a connection, via dedicated line 14 a, to theearth station node 6 a to which it is connected and, as before, the callis connected to the mobile terminal 2 a via the satellite 4.

Thus, in this embodiment, when a call from a local user originates froma different area (for example a different country), the call isinitially directed to the local satellite system gateway 8 a, which thendetermines the home gateway of the mobile user and directs the call onthrough the international public switched telephone network (PSTN).

B Mobile to Fixed Call

The process here is identical to that described in relation to FIG. 10,since the dial number supplied to the home gateway 8 a contains theinternational dialling prefix necessary to set up the call to the remotePSTN 10 b and called terminal 12 b; the first PSTN 10 a to which thegateway is connected will dial through as appropriate in accordance withthe dial number supplied by the mobile terminal 2 a.

3. OUT OF AREA LOCAL USER

As described above, if a locally registered terminal 2 is out of itsarea it will receive no service. In the event of an incoming call, asdescribed in relation to FIGS. 9 or 11, when the database station 15 isinterrogated, the reply signal will indicate that the status of theterminal equipment 2 is unavailable, and the call will be terminatedwithout it having been necessary to set up an international call. Whenthe mobile terminal 2 attempts to initiate an outgoing call, it will beunsuccessful.

4. GLOBAL USER

A Fixed to Mobile Call

Referring to FIG. 12, the initial steps in this process are as in thepreceding cases. The fixed terminal 12 b dials the dial number of themobile terminal 2 a. The local PSTN lob recognises it as pertaining tothe satellite service network and routes the call to the local gateway 8b. For example, the call number of the mobile terminal 2 a may beprefixed by an international dialling code, such that the call is routedthrough the PSTN 10 b to the international switch centre 20 b thereof,from whence it is routed to the gateway 8 b.

At the gateway 8 b, the gateway station transmits an interrogationmessage to the database station 15 including the call number oridentification of the called mobile terminal 2 a. The database station15 replies with a message which specifies the global status of themobile terminal 2 a, and comprises an identification of the earthstation node 6 a via which the mobile terminal 2 a may be contacted.

The gateway 8 b then sets up a connection via dedicated line 14 b to theactive earth station node 6 a, via its local node station 6 b, throughthe dedicated ground network lines 14. The active node station 6 a thensets up the link to the mobile terminal equipment 2 a via the satellite4 a as discussed above.

After completion of the call, the active earth station unit transmitsbilling data to the home gateway for the mobile user 2 a.

Thus, it will be seen that in this embodiment, in routing a call to aglobal rather than a local user, the gateway 8 at which the call firstarrives is arranged to route the call to the active earth station nodevia the dedicated ground network lines 14, rather than via the PSTN asfor a local user.

In this case, the home gateway of the mobile terminal equipment 2 aplays no part in the routing process; this is technically desirable,since the home gateway might be at a point on the planet distant fromeither the calling terminal 12 b or the active earth station node 6 a.Instead, the route from the calling gateway 8 b (which is generallygeographical close to the calling terminal 12 b) to the active earthstation node 6 a via the dedicated ground network lines 14 is chosen tobe relatively direct, and to utilise dedicated lines 14 of a qualitywhich will not, when in combination with the satellite link via thesatellite 4, render the link unacceptable to a user.

B Mobile to Fixed Call

Referring to FIG. 13, when a mobile terminal 2 a attempts to originatean outgoing call to a fixed terminal 12 b, it initiates a call via thesatellite 4 a to the earth station node 6 a, commencing by signallingthe dial number of the fixed terminal 12 b (including country code).

The earth station node 6 a determines, from the country code dialled,the gateway 8 b which is connected to (or is closest to) the PSTN 10 bto which the fixed terminal 12 b is connected, and sets up a circuitthrough the dedicated lines 14 forming the ground network to thatgateway 8 b via the earth station node 6 b to which it is connected.

The target gateway 8 b passes the remainder (excluding country code) ofthe dialled call number of the fixed terminal 12 b to the PSTN 10 b viathe international switch centre 20 b thereof and the call is routed tothe fixed terminal 12 b, the circuit being held open for the duration ofthe call.

On termination of the call, the active node 6 a transmits billing datato the home gateway of the mobile terminal apparatus 2 a as before.

Thus, it will be seen that in this case also, the call proceeds via thededicated ground network 14 rather than via the international PSTN lines21 (as was the case for the local registered user above), enabling abetter and relatively direct connection which does not involve the homegateway of the mobile terminal 2 a (which could be located distant fromthe mobile terminal or the fixed terminal).

MOBILE-TO-MOBILE CALLS

A mobile-to-mobile calls is executed simply as a mobile-to-fixed call(described above), followed by a fixed-to-mobile call (as describedabove). Thus, a call between two global users is routed entirely overthe ground network between the calling and called active earth stationnodes, as are calls from a global to a local mobile and vice-versa. Acall from one local user to another, on the other hand, is routedpredominantly through the PSTN.

LOCAL CALL NUMBERING

In the above-described embodiments the dial numbers allocated to mobileusers have ‘international’ prefixes followed by a code which does notcorrespond to any national PSTN but does correspond to the satelliteservice network.

It is equally possible, however, to provide an embodiment in which suchnumbers do have a prefix code which corresponds to a particular PSTN orPLMN. In this embodiment, the description is modified in the followingrespects. The local gateways 8 a etc. are connected to a local exchangeof the PSTN. When a fixed user dials a mobile user, the call is routedthrough the PSTN, either to the local exchange (if within the same PSTNas the fixed user) or through the ISC of the PSTN, via an internationalcircuit, to a distant PSTN within which the local exchange is located.

At the local exchange, the dial number is recognised as belonging to thesatellite mobile system and a call is set up to the local gateway 8 a.From this point on, the process is as described above, the call beingselectively routed either via the dedicated ground network or the PSTN(or PLMN or other network).

ACTIVE GATEWAY SELECTION

In the above described embodiments, the database station 15 stores anindication of the active earth station node 6 for each mobile userapparatus 2, based on a registration process which may be performed oninitiation of a call, on logging on, or repeatedly during a call.

In a further embodiment, the processor 58 of the database station 15 isarranged to select the active earth station node 6 on initiating of anincoming or outgoing call to the mobile user apparatus 2.

Referring to FIG. 14, a mobile user apparatus 2 a is positioned on theearth surface, and two satellites 4 a, 4 b are at a relatively highelevation angle with respect to the apparatus 2 a. Each of thesatellites 4 a, 4 b is also, as shown, at a relatively high elevationangle with regard to each of two earth stations 6 a, 6 b.

The apparatus 2 a can therefore be interconnected with anothercommunications system via one of the following routes:

2 a-4 a-6 a;

2 a-4 a-6 b;

2 a-4 b-6 a;

2 a-4 b-6 b.

In general, the signal quality via each of these links will bedifferent. In the example of FIG. 14, the best link quality may beobtained on the shortest uplink and downlink path, which is 2 a-4 a-6 a,and the worst may be obtained on the longest signal path which is 2 a-4b-6 a. The shortest path will have the shortest transmission time delay.Also, since it will involve the highest elevation angles, it will be theleast subject to attenuation by the atmosphere and terrain, and tomultipath effects.

Because of the relatively lower power or quality of the transmission andreception equipment available on the mobile user apparatus 2 a relativeto the satellite 4 a and the earth station 6 a, the signal quality maydepend predominantly on the link between the mobile apparatus 2 a andthe satellite 4 a. Preferably, as disclosed in our earlier applicationGB-A-2293725, communication is effected via a plurality of these linkssimultaneously.

Since the satellites 4 are travelling in known orbits and the positionsof the earth stations 6 are predetermined, the processor 58 is capableof calculating what the shortest mobile-satellite link and the shortestmobile-satellite-earth station link will be, for any given mobile userequipment position at any time of day.

Typically, several of the possible link paths between a mobile equipment2 and an earth station 6 may all be of acceptable quality. In this case,further factors may determine the selection of the active earth station.Firstly, the mobile terminal 2 may be registered as a “local” user andmay be entitled only to use one of the earth stations 6. In this case,this earth station is selected even if the link to this earth stationwould not be of such high quality as the link to another earth station6.

Furthermore, because of the motion of the satellites 4 (where they arein non-geostationary orbits) the preferred link may not be the link pathwhich is the shortest at the moment a call is initiated, because thesatellite in question may move away. Accordingly, this may lead to theselection of an Earth station which is arranged to communicate via adifferent satellite which will be in view for a longer period with acorresponding lower link quality, over a receding satellite albeithaving a higher link quality.

In the case of a mobile-to-mobile call, where a first mobile terminalapparatus is communicating via a first earth station 6, and that earthstation 6 is one of several earth stations via which links can beestablished to the called mobile terminal apparatus 2 a, it may bepreferred to select the first earth station so as to avoid the necessityto route the call through either the ground network or the terrestrialPSTN.

Finally, where other factors are equal, the selection of an active earthstation 6 amongst several possible active earth stations may be made independence upon the relative traffic carried by the two stations, so asto distribute traffic evenly between earth stations.

Referring to FIG. 15, a mobile terminal apparatus 2 a may be positionedon the earth surface adjacent to some shadowing structure 1000 such as,for example, mountainous terrain or a tall building. Under thesecircumstances, the path from the mobile terminal 2 a to the closestsatellite 4 a and earth station 6 a may be blocked.

However, as indicated, there may be a line of sight to a more distantsatellite 4 b at a lower elevation angle. The distant satellite 4 b maybe capable of communicating with the ground station 6 a which is nearestto the mobile terminal 2 a, but the path from the satellite 4 b to thenearest earth station 6 a may be long, and accordingly it is preferablefor the satellite 4 b to communicate via an earth station 6 b or 6 cwhich is more distant from the mobile terminal 2 a. Even of the twodistant earth stations 6 b or 6 c, the link quality to the earth station6 c which is further from the mobile terminal apparatus 2 a may bebetter than that to the nearer earth station 6 b.

Accordingly, in this embodiment, rather than merely relying upon eitherthe closest earth station 6 to the mobile terminal equipment 2 or theearth station 6 which is calculated, from the satellite ephemerides, tohave the best link quality, a test is additionally made of link qualityvia each of a plurality of earth stations before the active earthstation is selected.

CALL TO MOBILE

Referring to FIG. 16, the process performed by the processor 58 of thedatabase station 15 in response to a call, to a mobile user apparatuswill now be described. In a step 100, the processor 58 accesses thestore 54 and, in a step 102, reads the mobile position and status datafor the called mobile user apparatus from the store 54.

In a step 104, the control circuit 58 tests whether the status is‘local’ and, if so, in a step 106 the processor 58 calculates, using themobile position and stored satellite ephemeris data, whether the mobileuser apparatus 2 is positioned to be able to communicate with apermitted earth station (i.e. one associated with the geographical areaor part of the satellite communication system within which the localuser is permitted to travel).

If not, the call is terminated in step 108 by, for example, signallingthat the user is unobtainable (out of area). If the user is able tocommunicate with a permitted earth station (for example one connecteddirectly to a PSTN with which the user is registered), then this earthstations is allocated as the active earth station in a step 110.

If the status of the user is determined to be ‘global’, rather than‘local’, in the step 104, in a step 112 the processor 58 calculates theexpected signal quality (signal strength and delay time) on each of themobile terminal-satellite-earth station links which are possible, usingthe mobile terminal position and satellite ephemeris data, and selectsthe two best earth stations in a step 112.

Next, in a step 114, the database station sends a hailing signal via thesignalling link 60 to the two best earth stations calculated in the step112, and thence via a common signalling broadcast channel, via thesatellites with which each earth station is in communication, to themobile user equipment 2.

If the mobile user equipment 2 is switched on, it broadcasts in responsea reply signal on a common hailing frequency. The reply signal isreturned, via all satellites 4 in range of the mobile terminalequipment, to all earth stations 6 with which the satellites are incommunication. At the earth stations 6, the quality of themobile-satellite-earth station link is measured, to assess signalquality utilising for example the bit error rate, the frequency errorand so on.

Each earth station then signals a measure of the received quality back,via the signalling link 60, to the processor 58 at the database station15, which receives the reply signals in a step 116.

In a step 118, the measured link qualities are assessed, and the bestthree links are retained in a step 118, if they exceed minimum linkquality criteria. In a step 120, the control circuit 58 determineswhether the earth station (in the case of a call from another mobileuser terminal apparatus), or nearest earth station to the gateway (inthe case of a terrestrial originating call), from which the call came isone of the selected earth stations. In the event that it is, it isallocated as the active earth station in step 122.

This measure minimises the use which is made of the dedicated groundnetwork, by providing that wherever call quality is acceptable, only oneearth station is involved in handling the call.

In the event that the originating earth station (i.e. either the earthstation which receives the call in the case of a user originated call,or the earth station to which the originating gateway, is connected inthe case of the terrestrial originating call) is not selected in steps120 and 122, then in a step 124 the control circuit 58 interrogates eachof the selected earth stations in turn and receives a signal indicatinga current loading (call traffic volume) of the earth stations, in a step124. Any earth station which is too busy to process the call may not beselected as the active earth station (and if all earth stations arebusy, a signal indicating this is transmitted to the calling party).

If, following step 126, one or more earth stations is not busy then instep 132 the earth station via which the highest quality link isachievable is selected and allocated, in step 134, as the active earthstation.

Following allocation of an earth station in steps 110, 122 or 134, theaddress of the active earth station is signalled back to the originatinggateway or earth station as in the above described embodiments to allowthe call to be set up.

In the above process, as part of the link quality calculation step 112,it is possible also to take account of the nature of terrestrial linksbetween the originating gateway or earth station and each possibleactive earth stations, so as to avoid the selection of an active earthstation which is only accessible via a satellite link, for example.

OUTGOING MOBILE TO FIXED CALL

When a mobile user apparatus 2 is operated to initiate a call, itgenerates a hailing signal on a hailing channel, which is intercepted byone or more satellites 4. Each satellite relays the hailing signal tothe earth stations 6 to which it is in contact, which in turn measurethe received signal quality and forward the signal, together withsignals indicating the received quality, to the database station 15.

Referring to FIG. 17, in a step 202, the control unit 58 at the databasestation 15 receives the hailing and quality signals from each of theearth stations 6 and, in a step 204, accesses the store 54 in a step 204and reads the mobile position and status in a step 206. The mobileposition may be updated based on the signals received from the earthstations 6.

In a step 208, the status of the mobile is tested and, if the status is‘local’, in a step 210 the processor calculates (using the mobile userapparatus position data and satellite ephemeris data) whether theapparatus would have a predicted good quality link to the earth stationassigned to its geographical area, and measures whether the actual linkquality for that earth station is satisfactory.

If one or both of these conditions is not satisfied, the call isterminated in a step 212. If both conditions are satisfied, this earthstation is assigned as the active earth station in a step 214.

If the status of the user apparatus is determined to be global in thestep 208, the control circuit 58 determines, in a step 216, whether thedestination earth station (that is, the earth station via which the callwould be routed through the ground network to reach the gateway of thePSTN of the called number, or the gateway which is allocated or is to beallocated as the active gateway for a called mobile user apparatus 2) isa gateway station from which a good link quality signal has beenreceived, and furthermore is a gateway station for which, based on themobile position data and the satellite ephemeris data, a good predictedlink quality is calculated.

If both these criteria are met, in a step 218 the destination earthstation is allocated as the active earth station. If one or both ofthese criteria is not met, in a step 220, the processor 58 interrogatesthose earth stations from which signals were received (as in step 124above) to determine their state of busyness and, in step 222, determineswhich, if any, are busy (i.e. heavily loaded with call traffic).

In step 224, any earth stations which are already heavily loaded aredropped from further consideration. If all are fully loaded with trafficin step 226, the call is terminated. If one or more earth stations hascapacity, then in step 228 the control circuit 58 selects the earthstation which has the highest measured link quality.

Account may also be taken of the predicted future quality of the linkcalculated from the mobile user terminal position data and the satelliteephemeris data where several earth stations exhibit an acceptablemeasured link quality.

The earth station exhibiting the best link quality is allocated as theactive earth station in step 134.

It will be clear from the foregoing that, in this embodiment, varioustechniques which have been described in combination may be usedseparately of each other to achieve some advantages. For example, usemay be made exclusively of measured link quality ignoring satelliteephemerides, or may be made exclusively of a calculation of predictedlink quality based on user terminal position data and satelliteephemeris data, ignoring measured link quality.

Whilst it is convenient to allocate capacity according to the state ofbusyness of the earth stations, it is not essential to do so, and/orother methods of doing so may be employed.

The feature of preferentially employing the source (for an incomingcall) or destination (for an outgoing call) earth station as the activeearth station to serve the mobile user equipment (where this gives anacceptable link quality) is advantageous, but not essential.

Any or all of the above improvements may be used without the feature oflocal and global categories of users described in the foregoingembodiments.

OTHER EMBODIMENTS

It will be clear from the foregoing that the above described embodimentis merely one way of putting the invention into effect. Many otheralternatives will be apparent to the skilled person and are within thescope of the present invention.

For example, the numbers of satellites and satellite orbits indicatedare purely exemplary. Smaller numbers of geostationary satellites, orsatellites in higher altitude orbits, could be used; or larger numbersof low earth orbit (LEO) satellites could be used. Equally, differentnumbers of satellites in intermediate orbits could be used.

Although TDMA has been mentioned as suitable access protocol, thepresent invention is fully applicable to other access protocols, such ascode division multiple access (CDMA) or frequency division multipleaccess (FDMA).

Equally, whilst the principles of the present invention are envisagedabove as being applied to satellite communication systems, thepossibility of the extension of the invention to other communicationssystems is not excluded.

Although, for the sake of convenience, the term “mobile” has been usedin the foregoing description to denote the terminals 2, it should beunderstood that this term is not restricted to handheld or hand-portableterminals, but includes, for example, terminals to be mounted on marinevessels or aircraft, or in terrestrial vehicles. Equally, it is possibleto practice the invention with some of the terminals 2 being completelyimmobile.

Instead of providing a single central database station 15 storingdetails of all terminal equipment 2, similar details could be stored atthe home gateway 8 for all terminal equipment to register with that homegateway 8.

To reduce traffic volumes on the ground network, it would be possible toput the low bit-rate codecs at the gateways 8, so that all trafficwithin the satellite system is encoded, being coded on entry into anddecoded on exit from the system.

In the transmission of billing data, the billing data may either beactual cost or charge data, or duration data.

In the foregoing, the gateways 8 may in fact be comprised within an ISCor exchange or mobile switching centre (MSC) by providing additionaloperating control programmes performing the function of the gateway.

In the foregoing, dedicated ground networks lines have been described,and are preferred. However, use of PSTN or PLMN links is not excludedwhere, for example, leased lines are unavailable or Where temporaryadditional capacity is required to cope with traffic conditions.

It will naturally be clear that the stores within the gateways 8 neednot be physically co-located with other components thereof, providedthey are connected via a signalling link.

Whilst, in the foregoing, the term “global” is used, and it is preferredthat the satellite system should cover all or a substantial part of theglobe, the invention extends also to similar systems with morerestricted coverage (for example of one or more continents).

It will be understood that the geographical locations of the variouscomponents of the invention are not important, and that different partsof the system of the above embodiments may be provided in differentnational jurisdictions. For the avoidance of doubt, the presentinvention extends to any part or component of telecommunicationsapparatus or systems which contributes to the inventive concept ofselectively defining local and global system users, and treating the twodifferently

Furthermore, criteria other than user status may be employed to selectbetween the PSTN and the dedicated ground network; for example, trafficloading conditions may be taken into account as well as or instead ofuser status.

The foregoing, and all other variants, embodiments, modifications orimprovements to the invention are intended to be comprised within thepresent invention.

What is claimed is:
 1. A communication system for communication with aplurality of mobile terminals (2), the system comprising: a plurality oforbiting satellites (4); a plurality of earth stations (6) arranged tocommunicate with the mobile users via the satellites; a plurality ofgateway stations (8) for interconnecting terrestrial equipment with theearth stations (6); and a store (54) for storing access data for saidmobile terminals (2); characterised in that; the mobile terminals (2)are divided into first and second categories, the store (54) retainingcorresponding different status information, both said first and secondcategories communicating with said satellites (4), and in that there areprovided; first and second different communications channels (14; 10,21) between said gateway stations (8) and said earth stations (6), andin that there is provided; a route control device (72) for selecting oneof said first and second channels, in dependence upon the category of amobile terminal.
 2. A system according to claim 1, in which said firstcategory is a local category associated with a limited permittedgeographical range of said mobile terminal (2).
 3. A system according toclaim 1, in which said first category is a local category defining alimited permitted part of said system with which said mobile terminal(2) may communicate.
 4. A system according to claim 1, in which saidsecond category is a global category specifying that said mobileterminal (2) may communicate with any part of said system.
 5. A systemaccording to claim 1, in which said first communication channelcomprises a dedicated ground network (14) interconnecting said earthstations and said gateway stations.
 6. A system according to claim 5, inwhich said ground network (14) comprises leased lines.
 7. A systemaccording to claim 5 or claim 6, in which said ground network (14)comprises virtual circuits provided over shared lines.
 8. A systemaccording to claim 5, 6 or 7 in which said ground network (14) comprisesterrestrial lines in preference to satellite links.
 9. A systemaccording to claim 5, in which said ground network comprises trunkconnections (14 c) between said earth stations (6), and furthercomprises spur links (14 a, 14 b) from the earth stations (6) to thegateway stations (8) and in which there are provided routing switches(44) at said earth a stations.
 10. A system according to claim 1, inwhich said second channel comprises an international public switchednetwork link (21).
 11. A system according to claim 1, in which saidstore (54) is provided at one or more central storage stations (15),communicating with said earth stations (6) and/or said gateway stations(8) via a signalling channel (60).
 12. A system according to claim 1, inwhich said store (54) is arranged to store, for the mobile terminals(2), position information specifying the position of the mobileterminals (2).
 13. A system according to claim 1, in which said gatewaystations (8) are associated with terminal home stores (76), and saidmobile terminals (2) are each assigned to a said home store (76).
 14. Asystem according to claim 13, in which said home stores (76) arearranged to store billing data for mobile terminals (2) assignedthereto.
 15. A system according to claim 14, in which said billing datarepresents a different charge for said mobile terminals (2) of saidfirst category to that of mobile terminals (2) of said second category.16. A system according to claim 1, further comprising a device (58) foraltering the category of a mobile terminal (2), and for changing thestatus information held in the store (54) correspondingly.
 17. Satellitesystem routing equipment (8) for interconnecting a mobile terminal (2)satellite communications system (4, 6, 14) with a terrestrialcommunications link, said equipment comprising a routing switch (70) forselectively interconnecting said communications link with one of firstand second communications channels (14; 10, 21), either of whichconnects to said mobile terminal (2) via a communications satellite (4)and an earth station (6), depending upon whether said mobile terminal(2) falls into first or second predetermined categories.
 18. Equipmentaccording to claim 17, further comprising a terminal home store (76) forstoring billing data for a plurality of mobile terminals (2) associatedwith the home store (76).
 19. Equipment according to claim 18, furthercomprising means (74) for receiving billing data from said mobileterminal satellite communications system and for updating said homestore (76).
 20. Equipment according to claim 17, in which saidcommunications link comprises a telecommunications network (10), saidequipment comprising a gateway (8) between said telecommunicationsnetwork (10) and said mobile terminal satellite communications system,and said second communications channel (10) comprises the sametelecommunications network.
 21. Satellite system routing equipment (6)for interconnecting a mobile terminal (2), communicating via a satellitelink with said equipment (6), with a called terminal (12), the equipmentcomprising a routing switch (44) for selectively interconnecting saidsatellite link with one of first and second communications channels (14;10, 21), either of which connects to said called terminal (12),depending upon whether said mobile terminal (2) falls into first orsecond predetermined categories.
 22. Equipment according to claim 21, inwhich the routing switch (44) comprises an exchange for interconnectingportions of said first channel (14).
 23. Equipment according to claim21, comprising a satellite earth station (6).
 24. Equipment according toclaim 17 or claim 21, further comprising a signalling circuit forsignalling a routing request signal, indicating the identity of a mobileterminal (2) communicating on said satellite link, to a store (54) andfor receiving a reply signal, said equipment selecting either said firstor said second channel in dependence upon said reply signal. 25.Equipment according to claim 17 or claim 21, in which said firstcommunications channel comprises a dedicated ground network (14)connected to said earth station (6).
 26. Equipment according to claim25, in which said ground network (14) comprises a leased line. 27.Equipment according to claim 25, in which said ground network (14)comprises a virtual circuit provided over a shared line.
 28. Equipmentaccording to claim 17 or claim 21, in which said second communicationschannel comprises an international public switched network (21).
 29. Amethod of connecting a mobile terminal (2), via a satellite (4) and asatellite earth station (6), with a terrestrial communications link,comprising selecting one of two alternative routes (14; 10, 21) betweenthe earth station (6) and the communications link depending on whetherthe mobile terminal (2) is within first or second categories.
 30. Asystem for interconnecting a mobile terminal satellite commmnicationssystem with a terrestrial communications link, said system comprising:first and second communications channels; routing means for selectivelyinterconnecting said communications link with one of said first andsecond communications channels, either of said channels connecting tosaid mobile terminal via a communications satellite and an earthstation, said first communications channel connecting via the publicswitched telecommunications network and said second communicationschannel connecting via a satellite system ground network.
 31. A methodof interconnecting a satellite mobile communications system userterminal and another telecommunications user terminal, comprisingdetermining a plurality of possible earth stations via which saidsatellite system user terminal could communicate via a satellite;measuring received link quality for signals received from said satellitesystem user terminal via said plurality of earth stations: selecting oneof said plurality of earth stations according to at least said receivedlink quality; and interconnecting the two user terminals via saidselected earth station.
 32. A method according to claim 31 furthercomprising a step of determining the position of said satellite systemuser terminal (2).
 33. A method according to claim 31 in which saidearth station is further selected according to a predicted link qualitycriterion taking into account stored satellite ephemeris data.
 34. Amethod according to claim 31 in which the said other telecommunicationsterminal apparatus (12) is interconnected via one of said plurality ofearth stations (6), and further comprising a step of determining whethersaid one of said plurality of earth stations (6) will exhibit asatisfactory link quality as the earth station via which said satellitesystem user terminal (2) is accessed.
 35. A method according to claim 34in which a call is directed from said other telecommunications terminal(12) to said satellite system user terminal (2), said one of said earthstations (6) comprising an origin for said call.
 36. A method accordingto claim 34 in which a call is directed to said other telecommunicationsterminal (12) from said satellite system user terminal (12), said one ofsaid earth stations (6) comprising a destination for said call.
 37. Amethod according to claim 31 further comprising a step of assessingtraffic conditions via each of said plurality of earth stations (6). 38.A method according to claim 31 in which said satellite system userterminal (2) is associated with a predetermined category of a pluralityof said categories, and in which said selection depends upon said usercategory.
 39. Apparatus for selecting one of a plurality of earthstations for interconnecting a satellite mobile communications systemuser terminal and another telecommunications user terminal comprising:means for determining said plurality of possible earth stations viawhich said satellite system user terminal could communicate via asatellite; means for measuring received link quality for signalsreceived from said satellite system user terminals via said plurality ofearth stations; and means for selecting one of said plurality of earthstations according to at least said received link quality.
 40. A methodof interconnecting a satellite mobile communication user terminal andanother telecommunications user terminal, comprising the steps ofdetermining a plurality of possible earth stations via which saidsatellite system user terminal could communicate via a satellite, eachof said plurality of earth stations being connected to a dedicatedground network; selecting one of said plurality of earth stations; andinterconnecting the two user terminals via said selected earth stationthrough said dedicated ground network.